The women were about 41 percent more likely to have denser breast tissue than women who ate a Mediterranean-type diet.

"Generally, it is important to maintain an adequate weight through life by controlling caloric intake, reducing consumption of energy-dense foods," said study co-author Dr. Marina Pollan, a cancer epidemiologist at the National Center of Epidemiology in Madrid.

Women who ate such a diet had a 46 percent higher risk of developing breast cancer, Pollan added, although these study findings do not prove that diet causes breast tissue to become denser.

A Mediterranean diet is characterized by a high intake of fish, vegetables, legumes, boiled potatoes, fruits, olives and vegetable oil and a low intake of juices, she said.

"Women in the highest category of adherence to this pattern had a 44 percent lower risk of breast cancer than women in the lowest category of adherence," Pollan said.

For the study, Pollan and her colleagues collected data on more than 3,500 women who were part of a breast cancer screening program between October 2007 and July 2008.

The researchers collected medical information, family and personal health history and self-reported data about diet. They also rated the density of the women's breast tissue as seen on a mammogram.

The findings were adjusted for age, weight, menopause, smoking, family history, hormone treatment and calorie and alcohol intake, the researchers said.

The report was published Aug. 8 in the journal Obstetrics and Gynecology.

Dr. Stephanie Bernik is chief of surgical oncology at Lenox Hill Hospital in New York City. She said, "Mammographic density has been determined to be an increased risk factor for breast cancer."

This study suggests that a Western diet increases breast density. "This very well may be true, but more studies need to be carried out to ensure that there is not a different underlying cause that might be common amongst women with a diet high in fat and processed foods," Bernik said.

Perhaps these women are less likely to exercise, and this might be the true reason for the increased density, Bernik suggested.

A study needs to be designed to specifically look at diet and the effect on mammographic density, and not a study that makes observational conclusions based on a patient's memory, she said.

"Finding the root cause to the mammographic indicator of increased risk for breast cancer is very important," Bernik said.